Add missing `:` to the gtk-doc in a few places

Add missing `:` to the gtk-doc in a few places

This is SUCH a giant trap. I am not totally sure why it's
working for me in a fedora-42 build env, but it seems like
it may have broken in a different build environment in
https://github.com/ostreedev/ostree/pull/3548#discussion_r2500278890

I used Sonnet to audit for similar instances beyond
`read_blob` and it found some, fix those too.

Signed-off-by: Colin Walters <walters@verbum.org>